In order not to unduly complicate the following description, the terms "charger" and "accumulator" will be used herein to designate respectively such an apparatus and such a rechargeable electrical accumulator.
Moreover, and for the same reason, reference will solely be made to the charging of such an accumulator, even to designate the operation carried out after use of the accumulator for some time for the purpose of restituting the electrical energy supplied thereby, which operation is often termed recharging the accumulator.
In everyday language, the term "capacity" of an accumulator is very often used to designate what is in fact its nominal capacity, i.e. the magnitude, usually expressed in Amp.hours, which represents the quantity of electrical energy that the accumulator may theoretically restitute in given conditions, after having been fully charged. Therefore, in this sense, the capacity of a given accumulator has a fixed value.
It should however be noted that in the following description the term "capacity" will be used to designate the magnitude representing the quantity of electrical energy that an accumulator can restitute at any given instant, whereby the capacity is variable between a maximum value, which may be different from the nominal capacity of the accumulator, and a zero value, namely those values when the accumulator is fully charged and fully discharged.
The charging of an accumulator may be carried out by different methods which are well-known and which will not all be described here.
According to one of these methods, which is very often used, charging is carried out in two distinct consecutive stages.
During the first of these stages, the charger supplies to the accumulator a current Ia having a constant magnitude I1 which is selected in dependence on the nominal capacity on the accumulator and of the maximum admissible time for fully charging it after it has been discharged. Thus, for example, if one assumes that the charging of an accumulator having a nominal capacity of 200 Amp.hours may last ten hours, the selected value of magnitude I1 will be 20 Amps. This magnitude I1 must however in no case exceed a maximum value set by the manufacturer of the accumulator, otherwise there would be a risk of damaging the accumulator.
This first stage during which the voltage Ua across the accumulator's terminals increases regularly, terminates when said voltage Ua reaches a value U1 which is also set by the accumulator manufacturer.
Charging of the accumulator, which is still not complete at the end of this first stage, ends during the second stage, during which the charger maintains the voltage Ua across the accumulator's terminals at said value U1.
The current Ia then drops until it becomes very weak when the capacity of the accumulator has reached the maximum value.
A charger carrying out this method may consist of a source arranged so as to supply a constant current having the above-mentioned magnitude I1 as long as the voltage across its terminals is less than the above-mentioned value U1, and so that this voltage across its terminals does not exceed said value U1.
According to another method, which is also often used, charging of the accumulator is carried out in a single stage, by supplying to the accumulator a current Ia whose magnitude decreases progressively during the increase of the capacity of this accumulator, down to a very low value when the capacity reaches its maximum value, the voltage Ua across the terminals of the accumulator increasing during that time up to the above-mentioned value U1.
A charger carrying out this method may consist of a voltage source arranged to supply a no-load voltage having the value U1 and having an internal resistance such that the magnitude of the maximum current it can supply has the value I1.
It should be noted that the voltage Ua across the terminals of an accumulator should not exceed this value U1, otherwise there is a risk of damaging the accumulator and hence reducing its lifetime. However, this value U1 depends largely on the temperature of the accumulator during charging thereof.
Known chargers therefore often include means for adjusting this value U1 in dependence on the temperature of the accumulator, which means may be simply manual, or may be automatic in which case they of course include one or several temperature-measuring sensors arranged in the accumulator.
In the simplest known chargers, the value U1 is fixed, which means that, depending on the temperature, the accumulator is either not completely charged, or risks being damaged.
It is well-known than the various cells of an accumulator practically never all have the same maximum capacity. In other words, the maximum quantity of electrical energy that each of these cells may store during charging of the accumulator and later reinsure is practically never the same. This is due to the fact that it is practically impossible to manufacture these cells in such manner that the various parameters which determine their maximum capacity, for example the volume and/or the chemical composition of their active material, are rigorously identical from one cell to another. Moreover, some of these parameters may vary over time and/or with temperature.
It is also well-known that it is not desirable to cause a large current to continue to flow through a cell of an accumulator when this cell is fully charged, i.e. when its capacity has reached its maximum value, because this current then produces chemical and/or physical phenomena that may damage the cell. The seriousness of the damage to the cell in such a case increases with the magnitude of the current flowing through it, and this damage in particular results in further reducing its maximum capacity.
Now, when an accumulator is charged by means of a known charger, and whatever charging method is employed with this device, the same current evidently flows through all of the cells of this accumulator. As a result, that cell of the accumulator whose maximum capacity is lowest reaches this maximum capacity at a time when the other cells have still not reached their maximum capacity. As it is in general not possible to determine this time, the charging current of the accumulator continues to flow through the cell having the lowest maximum capacity after the latter has reached this maximum capacity. This cell is thus highly likely to be damaged by this current. As moreover the probable damage to this cell leads in particular to a reduction of its maximum capacity, each time the accumulator is charged again this cell further deteriorates little-by-little and in the end is completely destroyed, which leads to breakdown of the entire accumulator.
These drawbacks are all the more serious when the number of cells of an accumulator is great. Take for example the case of a lead accumulator charged by a charger carrying out the first above-described method.
For such an accumulator, the above-defined value U1 is for example set at 2.5 Volts per cell. If this accumulator includes sixty cells, this value U1 will be EQU U1=60.times.2.5 Volts=150 Volts
Let us also assume that one of the sixty cells, which will be called cell Ex to distinguish it from the fifty-nine other cells, has a maximum capacity less than each of the maximum capacities of these other cells, and that the maximum capacities of the latter are all equal.
During the first stage of charging this accumulator, the voltage across the terminals of cell Ex of course reaches the value 2.5 Volts before the voltage across the terminals of each of the other cells, at an instant designated by t1.
Let us further assume that at this instant t1 the voltage across the terminals of each of the fifty-nine cells other than cell Ex has a value of only, say, 2.3 Volts.
The voltage Ua across the terminals of the accumulator at instant t1 is thus only: EQU Ua=59.times.2.3 Volts+2.5 Volts=138.2 Volts
The charger thus does not interrupt the first stage of charging the accumulator and the magnitude of the current which continues to flow through the cells of the accumulator, including of course cell Ex, remains constant. The voltage across the terminals of each of these cells continues to increase, with the voltage across the terminals of cell Ex stabilizing quite quickly, when the capacity of cell Ex has reached its maximum value, at a value of say 2.7 Volts.
When the voltage Ua across the terminals of the accumulator reaches the above-mentioned value of 150 Volts at an instant designated as instant t2, and the charger interrupts the first charging stage of the accumulator, the voltage across the terminals of each of the fifty-nine cells other than cell Ex is thus equal to ##EQU1##
It is known that when a lead accumulator is charged by a constant current, as in the present example, the rate of increase of the voltage across the terminals of each of its cells is not constant in relation to time, but decreases as the voltage increases to become quite low when said voltage approaches the value 2.5 Volts. It can thus be understood that a fairly long time is needed between the instant t1, when the voltage across the terminals of all of the accumulator's cells, except for cell Ex, is equal to 2.3 Volts, and the instant t2, when said voltage is almost equal to 2.5 Volts.
There is thus a great risk of cell Ex being damaged by the large current which continues to flow through it for a fairly long time after its capacity has reached its maximum value.
In practice, because the maximum capacities of the accumulator's various cells are not all equal to one another, it is not just one cell which risks being damaged, but all those whose maximum capacity is relatively low and whose voltage reaches 2.5 Volts before the end of the first stage of charging accumulator.